Updating tiger to leopard good question for dating
Even if your Mac can run Lion or Mavericks, you'll need to upgrade to Snow Leopard first as an intermediary step, since Tiger and Leopard can't access the Mac App Store and can't perform in-place upgrades of OS X 10.7, 10.8, or 10.9.
Once you've installed your "new" operating system, revisit the sections above to see about moving to an even newer version that's still being actively supported.
First, stop using Apple's built-in apps where possible, the ones that have a connection to the Internet.
Just grab it from the Mac App Store and do an in-place upgrade—you shouldn't run into problems, though it is probably a good idea to run a Time Machine backup (or something similar) before doing it.
It's the first major security update that Snow Leopard has missed—the OS is still getting i Tunes updates, but its last major security patch happened back in September.